Neurodegenerative diseases, such as Alzheimer's and Parkinson's disease, are an increasing burden on our ageing society as there are currently no effective treatments for these disabling conditions. Treatment as well as early diagnosis of these and other diseases that originate in the brain remain challenging because the majority of suitable therapeutic molecules and diagnostics cannot penetrate the tight and highly restrictive blood-brain barrier (BBB) (Abbott, 2013). The BBB constitutes a physical barricade that is formed by brain endothelial cells (BEC) that line the blood vessels and connect with each other through tight junctions (Abbott, 2013). The tight junctions formed between the BEC are essential for the integrity of the BBB and prevent the paracellular transport of hydrophilic molecules larger than 500 daltons (Da). Because brain endothelial cells exhibit very low pinocytosis rates (Abbott, 2013), transcellular transport of larger molecules is limited to the highly specific receptor mediated transcytosis (RMT) pathway, and the passive, charge-based adsorption mediated transcytosis (Abbott, 2013; Pardridge, 2002). Additionally, the high density of efflux pumps, such as P-glycoprotein or the multi-drug resistance protein-1 (MDR-1), contribute to the removal of unwanted substances from the brain (Abbott, 2013).
While all these characteristics protect the brain from pathogens and toxins, they equally prevent the entry of most therapeutics. In fact, less than 5% of small molecule therapeutics and virtually none of the larger therapeutics can cross the BBB in pharmacologically relevant concentrations (i.e., sufficient to engage a central nervous system (CNS) target and elicit pharmacologic/therapeutic response) unless they are specifically ‘ferried’, that is, coupled to a transporter molecule. Due to the lack of effective ‘carriers’ to transport molecules across the BBB, numerous drugs against neurodegenerative diseases have been ‘shelved’ or eliminated from further development as they cannot be delivered to the brain in sufficient amount.
Different approaches to deliver larger molecules into the brain have been explored. For example, the integrity of the BBB may be disrupted, resulting in a leaky BBB, which in turn allows for unrestricted, paracellular entry of larger molecules into the brain. Tight junctions can be successfully loosened or disrupted by various approaches. For example, injection of substances that induce osmotic shock (for example, mannitol, hypertonic solutions) into the blood stream causes cell shrinkage and results in the disruption of tight junctions, therefore severely compromising the BBB (Guillaume, 2010). Other modulators of tight junctions include alkylglycerols, bradykinin and several analogues thereof, as well as viruses that modulate expression of proteins involved in maintaining the tight junctions (Erdlenbruch et al., 2003; Preston et al., 2008; Gan et al., 2013). A more localized disruption of the BBB is possible through application of ultrasound (Nhan et al., 2013). However, the periods during which the BBB is disrupted are sufficient to alter brain homeostasis and allow harmful chemicals, toxins and pathogens to enter the brain; this can result in serious side-effects, e.g., seizures and brain swelling, infection and possibly permanent neuropathological changes. Therefore, repeated treatments with these techniques for chronic and diffuse brain diseases affecting multiple brain regions are not practical. Most of these treatments are costly, necessitate hospitalisation, and some approaches require anesthesia.
Another approach for circumventing the BBB is direct injection of therapeutic molecules into the cerebrospinal fluid (CSF), the parenchymal space, or other parts of the brain. Several delivery methods have been developed, including: intracerebral (intra-parenchymal), intraventricular, and intrathecal delivery via infusion or convection-enhanced diffusion (CED) pumps. However, any type of direct injection into the brain or intracerebral implant is an invasive and costly procedure, as it requires hospitalization, anesthesia, and often surgery. Moreover, the poor diffusion rates of the therapeutics, particularly large biologics, within brain parenchyma limit the penetration of therapeutics to only small areas surrounding the site of injection/implantation. The correct placement of injections, catheters, and implants is challenging yet crucial to achieve diffusion of the drug to the targeted region of the brain. Additionally, catheters and implants provide a site for infection and/or immune response against the foreign material.
In another attempt to increase delivery across the BBB, CNS drugs have been modified to increase their brain uptake. Such modifications can include a change of their surface charge, a reduction in molecule size, and change to the lipohilicity of the drugs. However, any modifications to increase brain penetration are also likely to alter the overall pharmacology of the drug, including its desired activity and/or specificity. In addition, lipophilic molecules are more prone to being exported from the brain through the P-glycoprotein efflux pump.
Finally, endogenous transport mechanisms across the BBB have been exploited. Physiological mechanisms that allow transport of large molecules across the BBB can be divided into the highly specific receptor mediated transcytosis (RMT) and the non-specific charge based adsorptive mediated endocytosis pathways. Endocytosis is triggered upon binding of the specific ligand to its receptor, or upon electrostatic interaction between the cationic ligand or drug and the anionic functional groups on the brain endothelial cell surface (luminal side), respectively. Subsequently, the newly formed endosome is transcytosed across the cell to the abluminal side, to release its cargo.
Because adsorptive mediated transcytosis is non-specific, charge-mediated interaction, it occurs in all vascular beds and organs, limiting the availability of drug for brain delivery. Therefore, exploiting the RMT pathway remains the only physiological, non-invasive yet highly receptor-specific brain delivery method.
Only a few receptors are presently known to undergo RMT at the BBB and ‘ferry’ across their natural ligands: the well-studied transferrin receptor (TfR), the insulin receptor (IR), low-density lipoprotein receptor related proteins 1 and 2 (LRP-1 and -2) and diphtheria toxin receptor Peptides, natural ligands, and antibodies or antibody fragments have been developed that bind to these receptors (Pardridge et al., 1991; Yu et al., 2011; Muruganandam et al., 2001; Abulrob et al., 2005; Demeule, 2008; Sumbria et al., 2013), functioning as drug-to-brain transporters that utilize endogenous RMT pathways. Recently, antibodies against CD98hc, a component of the large neutral amino acid transporter (LAT1), have been shown to undergo transcytosis across the BBB (Zuchero et al., 2016), suggesting that this transporter could be another target for developing BBB carriers. However, to date only a single peptide (Angiopep ANG1005, targeting LRP-1) has been analyzed in phase II clinical studies, while other candidates are being studied in laboratory settings or are just entering Phase 1 studies. The RMT pathway appears to be the most promising pathway for transport of biologic drugs into the brain, but current approaches have limitations, including non-selective expression of the target receptor at the BBB, competition between the carrier and the natural ligands to the receptor, ineffective transcytosis of a receptor, as well as lysosomal degradation of endocytosed carriers (Xiao and Gun, 2013).
The lack of high-capacity and high-selectivity BBB carriers that do not disrupt the physiology and homeostasis of the BBB delays the development of new therapeutics and diagnostics for diseases originating in the brain, including brain tumors and neurodegenerative diseases.